Most syringes used today for medical, laboratory or dental purposes are sold as disposable items intended to be used only once. Disposal of such syringes has posed a safety hazard for the individuals who use the syringes as well as for those who dispose of them. With the onset of AIDS, the concern for infection due to accidental needle sticks from used syringes has increased and a number of different devices have been proposed to minimize the possibility of spreading infectious disease due to accidents of this type.
One approach to this problem is to provide an extendable needle shield which, after the syringe has been used, can be pushed to an extended position where it covers the needle, making it difficult for an individual to accidentally contact the needle. A common feature of such constructions is that when the needle shield is pushed to its extended position, it is locked so that it cannot be retracted (thus exposing the needle) except by application of extraordinary force.
A number of such constructions have been proposed to satisfy the general requirement that the needle be permanently covered after the syringe has been used. Some of these constructions involve complex locking mechanisms and, in others, locking occurs automatically when the shield is fully extended. These known devices satisfy many of the functional requirements of a needle shield but require, in most cases, modification of the standard syringe construction. This is highly undesirable for many manufacturers because of the sizable investment they may have already made in their existing molding equipment for producing the syringes. Constructions have also been proposed which include an opening or slot in the side of the shield. This is undesirable because the needle can extend through the opening if the needle or the needle shield is deflected when the needle shield is in the extended position. Other proposed devices include open ended needle shields which may not block access to the needle point by small fingers.
Those devices which lock in response to axial movement of the needle shield to the extended position, (i.e., without any rotation) have certain inherent drawbacks that result primarily from the requirement that substantial force be exerted axially to lock the shield in its extended position. In the first place, the likelihood of unintentional and irreversible locking is greater with such devices than with those devices which require rotation to lock. Also, in certain constructions, it may be difficult to verify that the needle shield is locked upon extension without attempting to retract the shield, which increases the possibility of unintended exposure of the needle.
In addition to the design requirements described above, the design of this type of dental syringe must allow for the insertion of a medication cartridge through the proximal end of the dental syringe. Additionally, the outer diameter of the distal end of the needle shield should be minimized so that the dentist is able to better observe the needle of the dental syringe during the use thereof and so that the distal end of the needle shield does not interfere with the placement or use of other instruments which may simultaneously be present in the patient's mouth.
Accordingly, it is an object of this invention to provide a protective needle shield of the type described above which can be added at minimal expense to conventional hypodermic or dental syringes.
Another object of the present invention is to provide a relatively inexpensive protective shield which satisfies the functional requirements of a needle shield and includes none of the drawbacks mentioned above.
Another object of the present invention is to provide an extendable needle shield for a dental syringe which performs all of the necessary functions of such a shield and which is particularly well suited to an automated process of manufacture.
A further object of the present invention is to provide an extendable needle shield for a standard hypodermic or dental syringe which is improved both from the points of view of functional utility and cost of manufacture.
A still further object of the present invention is to provide an inexpensive method of assembling a protective needle shield and dental syringe.